Overdrive in Automatic Transmission Working Principle
Automatic transmission vehicles have an overdrive gear which is the highest gear available. When you shift to select the overdrive gear, it will reduce the revolutions-per-minute of the engine as it is running. You will do this whenever you want to maintain a single speed on the road.
For instance, if your car has an overdrive mode that is activated, then your car will automatically shift to the overdrive gear after reaching around 50 miles per hour. You will notice when this happens because your engine won’t make as much noise as it normally does. And, since the RPM is reduced, your engine won’t require as much fuel either. This means you’re your fuel economy will be better as you remain in overdrive gear mode. Without the overdrive gear set, your car will continue to drive with one of the lower gears set and consume more fuel.
The Working Principle
When you go to select the overdrive gear with your shift stick, you won’t find it amongst the other letters on the gear panel. There is no “O” letter to represent overdrive that you would shift the stick toward. Instead, there is an overdrive button that is usually located on the side of the stick. You simply press this button to activate the overdrive gear. After you do this, the transmission downshifts throughout the other gears as it goes from the highest gear.
People who like to drive with cruise control on should take advantage of the overdrive gear option. It is only meant for driving at one set speed, but it must be a high speed. The reason is that it is easier for the engine to reduce its RPM if the vehicle is traveling at a higher consistent speed. In other words, when you activate the overdrive gear, the torque converter is locked so that the transmission absorbs all the engine power. This is how the engine will perform better and reduce its fuel consumption.
If you regularly drive on the highway or some other fast-moving road, you will benefit the most from the overdrive gear function. In automatic transmission vehicles that do not have an overdrive gear, the driver is forced to accelerate to speeds as high as 60 miles per hour while using the lower gears. This causes the engine to consume more fuel because it is under more strain and demand. That is why the RPM normally increases in these vehicles as you accelerate faster. However, in an automatic vehicle with an overdrive button, you can take this pressure away from the engine which lowers its RPM.
When Not to Use Overdrive
There is some debate about the lowest speed in which you can use the overdrive function. Typically, you will not want to activate the overdrive gear if you are driving at less than 50 miles per hour. Slower speeds like this will be harder to maintain if you have the overdrive gear selected. Instead of being one single speed, the speeds will shift erratically and turn quite inconsistent. This will make the driving conditions of the vehicle very dangerous. Therefore, if you are someone who normally drives throughout their town and city, then you will want to stay away from the overdrive gear until you reach the highways.
Read also: 3 Common Causes of Transmission Noise
How to Get Overdrive
The overdrive gear does not normally get installed by the car manufacturers. This is considered a special feature which people need to add on to their existing automatic transmission. There are overdrive kits that you can purchase for about $1,000 to $3,000. These kits will allow you to add-on the overdrive gear to your transmission. Contact a mechanic or your local auto shop to learn more about your options for an overdrive gear add-on.